Given that I've found this article, I'm assuming my problem is local, but since I'm casting about, I figure I'll see if anyone has info...

I was happily sharing my internet connection from my tower (a 400MHz G4) to my laptop (1GHz 17" Powerbook) when I had Jaguar installed on both, and continued to share with no problems after upgrading the laptop to Panther. However, since I upgraded the tower to Panther, I now have serious problems when I turn on Internet sharing over Airport. When I turn it on, my ethernet TCP/IP connection drops, and I can't even get it back by turning off internet sharing. I have to reboot the machine to get it back.